In general it works fine, though ehternet speed according to speedtest.net, fast.com and iperf with a server within my local network is capped by 30.5 Mbit/sec.

It's definitely a bug somewhere in DL hardware or software because when I connect a 3rd party gigabit usb ethernet dongle to a port on my DL dock I'm getting 250Mbit on speedtest/fast.com and 400Mbit over local network.

Could you help me with that please?.. In general it's nice piece of hardware but network performance is awful.

This is a known issue with DL-3xxx and DL-5xxx devices actually.
The built-in macOS Ethernet driver is very inefficient with those devices. We receive sporadic reports about this issue and never got Apple to analyse this issue unfortunately.

The issue is not visible with any other driver we tried on the platform or other platforms, including our own driver that we used to ship before Apple provided the inbox driver in macOS. Also, DL-6xxx devices are not affected.

We have bug #31218016 open with Apple about this, our internal reference is #25056. If you experience this issue, you could raise a bug with Apple too to increase its priority.

First time using this thing and noticed the network appeared to be lagging and sluggish. I pulled up the Network Utility and seeing all kinds of send errors. Doing an internet speed test, the send errors skyrocketed to over 30,000 for instance. I ruled out the cable, switch port and dock as being an issue. Other than that, everything appears fine.